There would unfortunately be no reason to have +18 lbs when there is +21 lbs for the Spitfire XIVe. This since both these manifold pressures were limited to 5 min usage according to the Spitfire XIV pilot handbook. As such the +18 lbs boost would just be a worse version of the +21 lbs boost with less horsepowers for no gain. It would also make the boost system more clunky since you'd have to scroll through three different boost stages instead of two. If the +18 lbs setting would instead have had a limit of 15 min then there would have been a reason to incorporate it for the Spit XIV flightmodel.
The only planes with three boost stages are where we simply couldn't make it into two stages. The Bf 109E-4 Aa is an example where 1.23 ata historically was limited to 30 min usage, 1.30 ata was limited to 5 min usage while 1.40 ata was limited to 1 min usage (in real life the Bf 109E's also had a 1 minute timer until automatic shutdown of 1.40 ata). As such the 1.30 ata and 1.40 ata can't be merged since it would be unfair to allow the Bf 109 to run with the horsepowers of 1.40 ata for 5 minutes, and it would also be unfair towards the Bf 109E pilot to only allow 1.30 ata for 1 minute. (1.40 ata was only usable up to ~1800 m, above that the pilot could only use 1.30 ata). As such we had to split it into a Bst1, Bst2 and Bst3, which isn't really preferable due to the clunkiness of the system, sometimes causing pilots to accidentally overheat their engine on Bst3 when they thought they went back to normal throttle (Max continuous).
<S> and I hope my post made sense